3. The small portion of vitrified matter, which was formed on the arm that supported the Gold in Mr. Homberg's experiment, may have come either from the arm itself, or rather from some heterogeneous particles contained in the Gold; for it is almost impossible to have it perfectly pure. 4. Neither Mr. Homberg, nor any that have repeated his experiment, ever reduced this pretended glass of Gold by restoring its Phlogiston, as is done with other metallic glasses. 5. To render the experiment decisive, the whole mass of Gold employed ought to have been vitrified; which was not the case. Nevertheless, I do not pretend that this metal is in its own nature absolutely indestructible, and unvitrifiable: but there is reason to think that no body hath hitherto found the means of producing those effects on it, probably for want of a sufficient degree of fire; at least the point is very doubtful. Gold cannot be dissolved by any pure acid: but if the acid of nitre be mixed with the acid of sea-salt, there results a compound acid liquor, with which it has so great an affinity that it is capable of being perfectly dissolved thereby. The Chymists have called this solvent _Aqua Regis_, on account of its being the only acid that can dissolve Gold, which they consider as the King of Metals. The solution of gold is of a beautiful orange colour. If Gold dissolved in _aqua regis_ be precipitated by an alkali or an absorbent earth, the precipitate gently dried, and then exposed to a certain degree of heat, is instantly dispersed into the air, with a most violent explosion and noise: Gold thus precipitated is therefore called _Aurum Fulminans_. But if the precipitated Gold be carefully washed in plenty of water, so as to clear it of all the adhering saline particles, it will not fulminate, but may be melted in a crucible without any additament, and will then appear in its usual form. The acid of vitriol being poured on _aurum fulminans_ likewise deprives it of its fulminating quality. Gold does not begin to flow till it be red-hot like a live coal. Though it be the most malleable and most ductile of all metals, it has the singular property of losing its ductility more easily than any of them: even the fumes of charcoal are sufficient to deprive it thereof, if they come in contact with it while it is in fusion. The malleability of this metal, and indeed of all the rest, is also considerably diminished by exposing it suddenly to cold when it is red-hot; for example, by quenching it in water, or even barely exposing it to the cold air. The way to restore ductility to gold, when lost by its coming in contact with the vapour of coals, and in general to any metal rendered less malleable by being suddenly cooled, is to heat it again, to keep it red hot a considerable time, and then to let it cool very slowly and gradually; this operation frequently repeated will by degrees much increase the malleability of a metal.